In a conventional vehicle having an automatic transmission, a driver manually selects a desired transmission range using a multi-position Park, Reverse, Neutral, Drive, and Low (PRNDL) lever. For instance, the driver could move the PRNDL lever to a drive (D) position to thereby select a forward drive operating range. A typical PRNDL lever is mechanically coupled to a detent lever and a manual valve assembly, usually via a length of cable. Movement of the PRNDL lever rotates the detent lever, which in turn shifts the transmission to the selected operating range. Shift logic thereafter automatically controls the progression of gears occurring during any required gear shift within the selected operating range.
Alternative shift-by-wire transmission designs are characterized by an absence of a direct mechanical linkage between the PRNDL lever and the detent lever/manual valve assembly. Transmission operating range selection in a by-wire design instead occurs by way of electronic control signals transmitted to a drive motor or other downstream actuator. The drive motor is connected to the detent lever, and responds to the electronic control signals by rotating the detent lever to the selected operating range position.